Measurement of differential collisional excitation cross sections for the K$\alpha$ emission of He-like oxygen

TL;DR

This study measures the energy-differential cross sections for Kα emission in He-like oxygen using an electron beam ion trap, validating theoretical models and enhancing plasma diagnostics accuracy.

Contribution

It introduces a novel method to extract cross sections near excitation thresholds and compares experimental data with advanced theoretical calculations.

Findings

Validated R-matrix and distorted-wave calculations against measurements

Demonstrated effective subtraction of forbidden line contributions

Supported use of state-of-the-art atomic codes for diagnostics

Abstract

We measure the energy-differential cross sections for collisional excitation of the soft X-ray electric-dipole K$\alpha$ ($x+y+w$) emission from He-like oxygen (O VII), using an electron beam ion trap. Values near their excitation thresholds were extracted from the observed emissivity by rapidly cycling the energy of the exciting electron beam. This allows us to subtract time-dependent contributions of the forbidden $z$-line emission to the multiplet. We develop a time-dependent collisional-radiative model to further demonstrate the method and predict all spectral features. We then compare the extracted $x+y+w$ cross-sections with calculations based on distorted-wave and R-matrix methods from the literature and our own predictions using the Flexible Atomic Code (FAC). All R-matrix results are validated by our measurements of direct and resonant excitation, supporting the use of such state-of-the-art codes for astrophysical and plasma physics diagnostics.